1. Field of the Invention
The present invention relates to an active matrix organic light emitting display and driving method thereof, and more particularly, to an active matrix organic light emitting display having a pre-charge current source and driving method thereof.
2. Description of the Prior Art
Flat panel displays have advantages such as low power consumption, no radiation and thin appearance, and have therefore gradually replaced traditional cathode ray tube (CRT) displays. Various kinds of flat panel displays have been developed to offer consumers better products. Among them, organic light emitting diode (OLED) displays have gained more and more attention due to their characteristics such as self-emitting light source, high brightness, high contrast, high emission rate, fast reaction, wide viewing angle, and low power consumption.
An OLED is a current-driven device whose luminance is determined by the driving current passing through the OLED. By controlling the value of the driving current, images having different brightness (or different gray scales) can be displayed. OLED displays can be categorized into passive matrix organic light emitting diode (PMOLED) displays and active matrix organic light emitting diode (AMOLED) displays according to the driving methods. In a PMOLED display, pixels on different rows/columns (scan lines/data lines) are driven sequentially. The luminance of each pixel is thus limited by the scan frequency and the number of the scan lines. Therefore, the PMOLED displays are mainly used in small-sized and low-resolution displays. In an AMOLED display, each pixel has a separate pixel circuit comprising a storage capacitor, an OLED and two thin-film transistors (TFTs). The pixel circuits can control the amount of current supplied to corresponding OLEDs. Therefore, the AMOLED displays can achieve uniform display characteristics by supplying a stable driving current to each pixel, and are particularly suitable for applications in large-sized and high-resolution displays.
FIG. 1 shows a diagram of a prior art AMOLED panel 10. The AMOLED panel 10 includes a data line DL, a scan line GL, and a pixel circuit 100. The pixel circuit 100 includes an OLED 110, a storage capacitor 120, TFTs 130 and 140, and voltage sources Vcc and Vss. The TFT 130 includes a gate coupled to the scan line GL and a drain coupled to the date line DL. The TFT 140 includes a gate coupled to a source of the TFT 130 and a drain coupled to the voltage source Vcc. The storage capacitor 120 is coupled between the source of the TFT 130 and ground, and the OLED 110 is coupled between the source of the TFT 140 and the voltage source Vss. When displaying an image, a scan signal is sent to the TFT 130 via the scan line GL for turning on the TFT 130, thereby coupling the storage capacitor 120 to the data line via the TFT 130. Also, current from the data line charges the storage capacitor 120 and a gate voltage required for turning on the TFT 140 is stored in the storage capacitor 120. Once the TFT 140 is turned on, a current IOLED flows through the OLED 110, whose luminance is determined by the value of the current IOLED. The current IOLED can be represented by the following formula:IOLED=½μ·COX·W/L·(VGS−VTH)2; where
μ is the electron mobility;
COX is the gate oxide capacitance per unit area of the TFT 140;
W is the channel width of the TFT 140;
L is the channel length of the TFT 140;
VTH is the threshold voltage of the TFT 140; and
VGS is the voltage difference between the gate and the source of the TFT 140.
The gray scales of images displayed by the pixel circuit 110 is determined by the value of IOLED, which is controlled by the voltage VGS based on charges stored in the storage capacitor 120. When displaying an image of a low gray scale, the pixel circuit 100 requires a small current IOLED. To generate a corresponding small voltage VGS, the current sent from the data line for charging the storage capacitor 120 is also small. Under this circumstance, the small current cannot efficiently charge the storage capacitor 120 for providing a sufficient voltage VGS, and the pixel circuit 110 might not be able to completely display the image having the required low gray scale. Therefore, the prior art AMOLED displays have poor display quality when displaying images of low gray scales.